United Nations Secretary-General Ban Ki-moon appealed for the release of all missing and detained staff on Friday and urged that those responsible for killing a U.N. official in Iraq and others be brought to justice.

In Syria alone, the U.N. said 35 staff members are missing or detained, the majority from the agency for Palestinian refugees.

Ban said in a message marking the International Day of Solidarity with Detained and Missing Staff on Friday that five U.N. personnel are being held by unnamed member states “without any reasons given for the arrests.” He said six staffers were held hostage by armed groups last year before being released.

“Personnel, especially those deployed under dangerous conditions, deserve full protection and rights,” the secretary-general said. “I call on all parties to respect the rights, privileges and immunities of United Nations personnel.”

Ban singled out the killing of Amer al-Kaissy, an Iraqi who headed the U.N. Development office in Diyala and was abducted outside the government building in Baqubah on April 26, 2015. His body was found and finally identified last month.

In 2014, the U.N. reported that 128 U.N. civilians were detained or arrested, and only a minority involved legitimate national criminal charges, the U.N. Staff Union said.

It urged Ban to establish a high-level panel to review security and laws related to U.N. personnel who are “increasingly facing deliberate attacks, kidnapping, abductions and unlawful detentions.”

Kevin Kennedy, the U.N. regional humanitarian coordinator for the Syria crisis, said that since March 2011 when the conflict began, 17 U.N. staff members have lost their lives.